1. Field of the Invention
The present invention relates to a transport system having multilayer tracks and a controlling method thereof. In particular, the present invention relates to a transport system that is applied in semiconductor fabrication to improve the transporting efficiency.
2. Description of Related Art
In a semiconductor fabrication plant, production materials are often stored at different manufacturing bays waiting to be processed, or kept at the same manufacturing bay on multiple occasions for undergoing particular fabrication steps. Modern semiconductor fabrication plants typically employ automated transport systems for more precise and efficient transporting of the production materials. The main objective is to assist in moving the production materials to the manufacturing bay, or for moving the treated production materials or end products from the manufacturing bay to a specific storage location. Particularly, transport systems may be used for moving the production materials stored at different storage locations or in between stations.
In traditional, OHT (Overhead Hoist Transport) system is used to transport the production materials and raw materials in the manufacturing bay, for example the production materials and raw materials are transported between storing region and the manufacturing tools. Typically, OHT system usually relies on ceiling rails arranged above the floor of the working area to move shuttle carriages.
On the other hand, OHS (Overhead Shuttle) system provides higher transport-rate and higher transport loading than OHT system. In addition, the height of the ceiling rails of OHS system is usually higher than that of OHT system.
However, in order to transport object between OHS system and OHT system, at least one transferring station (so-called as Stocker), which is similar with the elevator in mechanical characteristics, is installed between the rails of OHS system and OHT system. The transferring station is inadequate for transporting large amount of object because the low transporting efficiency which is resulted from the long transportation passage through the transferring station. Moreover, after transporting the object cross the manufacturing bays, the original shuttle carriage is defined as an empty shuttle carriage. The empty shuttle carriage cannot carry any object until moving back to the original manufacturing bay. In other words, the empty shuttle carriage issue limits the transport loading because the empty shuttle carriage cannot be designated to carry object.